1. Field of the Invention:
This invention relates to distance meter built-in focusing screens for optical instruments, and, more particularly, to improvements of focusing screens of the so-called Duo Focus type where on the focusing screen there is provided a cylindrical lens as the distance meter the ability of that cylindrical lens to increase and decrease the size of image in one direction in out-of-focus condition is utilized in measuring the object distance (focusing, distance coincidence).
2. Description of the Prior Art:
The distance measuring principles of a cylindrical lens are first explained below by reference to FIGS. 1 and 2.
In FIG. 1, an optical instrument is equipped with an image forming objective lens 1 in front of a focal plane in which a focusing screen 2 lies. The focusing screen 2 is provided with a distance meter in the form of a cylindrical lens 3 at the center of the area thereof with its mother line in parallel with the surface of the focusing screen. In order measure the distance, the objective lens 1 is axially moved while the area of the cylindrical lens 3 is observed from the rear of the focusing screen 2 by an eye 4 of the operator.
In FIG. 2, letting 0-0 denote the optical axis of the cylindrical lens 3, r the radius of curvature of the cylindrical lens with the center at a point A, and f the focal length of the cylindrical lens 3, the focal length f for the thin lens system may be expressed as follows: EQU f=r/(n-1)
where n is the index of refraction of a material of which the cylindrical lens 3 is made up.
Now assuming that a ray of light L from the objective lens 1 (FIG. 1) enters the cylindrical lens 3 and arrives at a point B on the rear or convex surface thereof, the distance of the point B from the optical axis 0-0 being h, then the ray L exits from the surface in deflection by an angle, meeting the optical axis 0-0 at the point C which is the focal point of the cylindrical lens. This deflection angle may be defined by .delta.=h/f, thus being proportional to the height h of incidence of that ray.
With the principal plane of the cylindrical lens 3 coinciding with the focusing screen surface, when the objective lens 1 takes a focusing position where a sharp image is formed at a plane deviated from the screen surface either rearwardly or forwardly by a distance, dx, (the object is out of focus), the image viewed through the cylindrical lens 3 appears to be shifted in a direction perpendicular to the optical axis 0-0 by an amount, dh. This amount of shift, dh, is: EQU dh=.delta.dx=(h/f)dx
thus being proportional to the distance h of the image point from the optical axis and the defocusing amount, dx, and inversely proportional to the focal length of the cylindrical lens 3.
That is, in the rear defocusing position, the point B appears to lie at a point D so that a subject of length, h, is observed as reduced to a length, h.sub.1. Conversely in the front defocusing position, the point B appears to lie at a point E so that the length, h, is observed as enlarged to a length, h.sub.2. In conclusion, the cylindrical lens 3 has a function of shortening or enlarging the off-axis amount, h, of the out-of-focus image. This function is valid only for the direction perpendicular to the cylindrical axis of the lens 3, and has no effect in the axial direction.
With such cylindrical lens 3 provided on the focusing screen 2 at the center of the area thereof, disposed with its mother line in parallel to the focusing surface of the screen, and oriented so that the mother line makes an angle of, for example, 45.degree. with the horizontal line as shown in FIG. 3, therefore, when focusing of the objective lens 1 is performed, while the field of view of the focusing screen 2 is observed, an image 5 of an object of which the distance is to be measured (for convenience, the object is assumed to be a vertical line) formed by the objective lens 1 appears upon detection of an in-focus condition to be a single continuous line, since an image section 5.sub.2 in the cylindrical lens 3 coincides with upper and lower image sections 5.sub.1 and 5.sub.3, as shown in FIG. 3(b). Unless the objective lens is focused on the object, the observer will recognize a shifted image with the image section 5.sub.2 in the cylindrical lens area to be inclined with respect to the upper and lower image sections 5.sub.1 and 5.sub.3 as shown in FIG. 3(a) and FIG. 3(c) for front and rear defocusing positions respectively. Thus, this type distance meter is characterized in that when the object is out of focus, the image section 5.sub.2 seen in the cylindrical lens 3 appears as turned by an angle proportional to the increase or decrease of the above identified off-axis distance, h.
It is noted that instead of the cylindrical lens of the convex type, a concave one may be used to effect the above described phenomenon except that the direction of deformation of the image to the shortening and enlarging with defocusing is reversed to that when it is of the convex type.
To achieve distance measurement, therefore, the operator is required to adjust the position of a focusing actuator of the optical instrument until the three image sections merge into one. The foregoing is the outline of the distance measuring principles.
In putting the principles into practice, the focusing screen is constructed by the use of one or more cylindrical lens or lenses on the focal plane of the screen plate. In the latter connection, it is known (A) to arrange the cylindrical lenses in spaced relation to each other, and (B) to arrange the cylindrical lenses in adjacent parallel relation to each other with their adjoining lines being collimated to the mother lines of the cylindrical lenses.
The focusing screen of the type (A), because of its individual cylindrical lenses admitting mere comparison of the image sections seen by the cylindrical lenses with those seen in the surrounding areas of the focusing screen, has a disadvantage of providing as low a distance measurement accuracy as when only one cylindrical lens is used. The focusing screen of the type (B), though being superior in distance measurement accuracy to that of the type (A), has a drawback that when the optical instrument is associated with an objective lens of dark F-number, a black veil is formed around the boundary of each pair of cylindrical lenses. This makes it difficult to perform distance measurement while seeing the object at the veiled area thereof. This leads to decrease of the distance measurement accuracy.